In particular, the invention may be applied to manufacturing an optical fiber on a fiber-drawing lathe. It is known that the fiber is produced from a preform or fiber-drawing cone that is heated. The resulting fiber passes successively through two resin-coating dies so as to provide the fiber with a certain amount of strength. The fiber is then wound onto a capstan applying a certain amount of tension to the fiber. The fiber-manufacturing assembly is disposed vertically.
It is necessary for the two dies to be centered accurately on the fiber so that it is coated properly. If the fiber touches a die, the thickness of the coating is not properly uniform around the entire periphery of the fiber, and the rate of fiber breakage increases.
Currently, a laser is used for centering dies, but given the size of the dies, diffraction occurs and that method is not very satisfactory. The diameter of a helium-neon laser beam is about 0.8 mm, whereas the first die has a diameter of about 0.24 mm, and the second die has a diameter of about 0.32 mm. To avoid diffraction, the diameter of the hole needs to be three or four times the diameter of the light beam.
Another method used relies merely on operator skills, and often means that the alignment achieved is not repeatable.